The present disclosure relates generally to methods for forming activated carbon, and more particularly to microwave energy-assisted, physical activation of carbon using a fluidized bed reactor.
Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.
Carbon-based electrodes suitable for incorporation into high energy density devices are known. The carbon materials, which form the basis of the electrodes, can be made from natural or synthetic precursor materials. Natural precursor materials include coals, nut shells, and biomass. Synthetic precursor materials typically include phenolic resins. With both natural and synthetic precursors, carbon materials can be formed by carbonizing the precursor and then activating the resulting carbon. The activation can comprise physical (e.g., steam) or chemical activation.
Chemical activation processes, which use an activating agent such as KOH, are typically more expensive than physical activation processes due to the added cost of the chemical activating agent raw materials and the attendant costs associated with processing and handling the same. For instance, chemical activation processes can involve the generation of corrosive bi-products, which require abatement solutions that can add to the process cost. Physical activation with steam, on the other hand, is typically more environmentally friendly and thus can be advantageous when considering the direct and indirect costs associated with the raw materials. However, steam activation processes typically use conventional furnaces to heat the reactants, which involve long activation cycle times at high temperatures.
Accordingly, it would be an advantage to provide activated carbon materials and processes for forming activated carbon materials using a more economical steam activation route. The resulting activated carbon materials can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.